This proposal deals with several approaches which will help to define the complicated steps involved in avian erythropoiesis. One of the novel aspects of this research involves isolation of many different genes (all of which are related to red cell development) from a single recombinant DNA library, a set of DNA sequences representing the entire chicken genome (constructed by random shear of chick erythroid DNA and subsequent formation of recombinant phage). The diversity of the library methodology will be examined and exploited in these studies. Globin gene expression is controlled by two types of regulation; one involves complete on/off switching from embryonic to adult gene types, whereas the other involves modulation in levels of expression throughout development. The central question we address (by restriction enzyme, electron microscopic, DNA sequencing and in vitro translation analyses) is: how is the temporal response of developmentally controlled gene expression elicited? Another aspect of this proposal focusses on the role of histone H5 in chicken erythropoiesis. Histone H5 is subject to very different regulatory mechanisms than the other histones. It is produced only in red blood cells (a terminally differentiated cell) and is uncorrelated with DNA synthesis. Thus the role of histone H5 in red cell development will be a point of some interest. We will use much the same approach we have taken in the globin gene studies; that is to examine the sequence arrangement of the histone genes by isolation of recombinant chromosomal DNA clones from the bacteriophage library. The final aspect of this proposal deals specifically with the expression of genes involved in chick erythropoiesis. Emphasis will be centered on the maturation of nuclear globin RNA precursors to cytoplasmic mRNA, the host-mediated response of cultured chicken cells to RNA tumor virus infection, and in vitro alteration of the cellular specificity of globin gene expression.